Niv-Mizzet, Supreme doesn't apply Jump-start correctly to spells in your graveyard.

A card like Far // Away is 2 colors in the graveyard, but on the stack is only one color, and when being evaluated for being legal to play it should be illegal. Arena allows you to play Far or Away from the graveyard with Niv-Mizzet, Supreme out.

A card like Push // Pull is 3 colors in the graveyard, but on the stack is exactly 2 colors, one half or the other. When it's evaluated it should be legal to play.

In both cases the Jump-start ability is shown to be added to the both halves of the cards, but only Far // Away is castable within Arena.